Injection pumps for a two-stroke engine in a work apparatus utilize the crankcase pressure as a drive. With the downward movement of the piston in the direction of bottom dead center, an overpressure is built up in the crankcase while, for a subsequent upward movement of the piston, the crankcase pressure drops to an underpressure. With pending overpressure in the pulse chamber, the pump piston plunges into the pump chamber by means of the membrane plate whereby fuel is injected into the combustion chamber of the two-stroke engine. With pending underpressure, the pump piston moves under the action of the spring into its start position determined by the stop. The pump piston stroke carried out in this manner is directly proportional to the injected fuel quantity.
It is known to make a greatly enriched mixture available in the combustion chamber when the engine is cold which is then leaned to an optimal mixture after the warm-up phase of the engine.